The present invention relates to a magnetic recording medium and a coating composition therefor containing fine particles of a ferromagnetic material dispersed therein as well as a vehicle resin as a binder of the ferromagnetic particles therein. More particularly, the invention relates to an improvement of the vehicle resin in a magnetic coating composition for forming a magnetic coating layer on a base material of a magnetic recording medium to impart excellent performance thereto with very high binding power for the ferromagnetic particles.
As is known, magnetic recording media such as magnetic recording tapes, floppy disc sheets and the like are prepared by providing a film or sheet-like substrate or base material of polyester resins and the like on one of the surfaces with a magnetic coating layer by coating with a magnetic coating composition comprising fine particles of a ferromagetic material and a vehicle resin to serve as a binder of the ferromagnetic particles dispersed and dissolved in an organic solvent. Various kinds of ferromagnetic materials are used as the ferromagnetic powder in the magnetic coating composition including powders of iron oxides, e.g. .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, iron oxide powders doped with cobalt ions or iron oxide powders on which cobalt ions are adsorbed, needle-like fine particles of powders of ferromagnetic metals such as iron, cobalt and alloys of iron and cobalt optionally containing nickel.
Along with the trend in recent years toward upgrading of the performance of the video and audio instruments, the magnetic recording tapes used in these instruments are also required to have a high density of recorded signals and high output in playing back of the recorded signals, in particular, in short wavelength recording. It is essential in order to comply with these requirements that the powder of the ferromagnetic material should be pulverized more and more finely in addition to the improvement in the magnetic properties thereof. Accordingly, the ferromagnetic particles have an extremely large magnetic moment so that the particles readily form agglomerates which can hardly be dispersed with sufficient uniformity in the vehicle resin of the magnetic coating composition.
Extensive investigations have been undertaken to solve the above described problem of poor dispersibility of the ferromagnetic particles in the vehicle resin of a magnetic coating composition from the standpoint of affinity between the ferromagnetic powder and the vehicle resin. For example, some of the vehicle resins conventionally used in magnetic coating compositions are vinyl chloride-based ones having carboxyl and hydroxy groups in the molecular structure thereof although the dispersibility of ferromagnetic particles therein is not quite satisfactory so that the magnetic coating layer has poor surface properties not to impart the magnetic coating layer with full magnetic performance such as residual magnetic flux and squareness ratio of the hysteresis curve in addition to the low durability of the coating layer due to dusting frequently taking place.
In order to improve the dispersibility of the ferromagnetic particles in the vehicle resin and to increase the smoothness of the surface of the magnetic coating layer, alternatively, various attempts and proposals have been made hitherto including addition of a surface active agent or a silane coupling agent having reactivity with the vehicle resin to the coating composition, surface treatment of the ferromagnetic particles beforehand with a silicone fluid and so on, although none of these prior art methods is quite satisfactory. Accordingly, it has been eagerly desired to develop a novel and improved magnetic coating composition capable of giving a magnetic recording medium free from the above described problems and disadvantages in the prior art products by virtue of the use of a specific vehicle resin in the coating composition so as to give good dispersibility of the ferromagnetic particles therein.